Extrusion of polymeric materials in the formation and shaping of articles is a major segment of the plastic or polymeric articles industry. The quality of the extruded article and the overall success of the extrusion process are typically influenced by the interaction of the fluid material with the extrusion die. For any melt-processable thermoplastic polymer composition, there exists a critical shear rate above which the surface of the extrudate becomes rough or distorted and below which the extrudate will be smooth. See, for example, R. F. Westover, Melt Extrusion, Encyclopedia of Polymer Science and Technology, vol. 8, pp. 573-81 (John Wiley & Sons 1968). The desire for a smooth extrudate surface competes with, and must be optimized with respect to, the economic advantages of extruding a polymer composition at the fastest possible speed (for example at high shear rates).
At low shear rates, defects in extruded thermoplastics may take the form of “sharkskin”, which is a loss of surface gloss that in more serious manifestations appears as ridges running more or less transverse to the extrusion direction. At higher rates, the extrudate can undergo “continuous melt fracture” becoming grossly distorted. At rates lower than those at which continuous melt fracture is first observed, certain thermoplastics can also suffer from “cyclic melt fracture”, in which the extrudate surface varies from smooth to rough.
There are other problems often encountered during the extrusion of thermoplastic polymers. They include a build-up of the polymer at the orifice of the die (known as die build up or die drool), high back pressure during extrusion runs, and excessive degradation or low melt strength of the polymer due to high extrusion temperatures. These problems slow the extrusion process either because the process must be stopped to clean the equipment or because the process must be run at a lower speed.
The addition of fluoropolymers can at least partially alleviate melt defects in extrudable thermoplastic polymers. Fluoropolymers that can be used as polymer processing additive include those described, for example, in U.S. Pat. Nos. 5,015,693 and 4,855,013 (Duchesne et al.), U.S. Pat. No. 5,710,217 (Blong et al.), and U.S. Pat. No. 6,277,919 (Dillon et al.). Some benefits of polymer processing additives include the elimination of melt fracture and surface defects, a reduction in extrusion pressure, and the elimination of die build-up. The incorporation of acidic end groups has been proposed to be beneficial to the interaction between the polymer processing additive and the die wall. See, e.g., U.S. Pat. Pub. No. 2011/0172338 (Murakami et al.) and U.S. Pat. No. 5,132,368 (Chapman et al.). On the other hand, acidity in the backbone has been proposed to be detrimental to chemical stability. See, e.g., U.S. Pat. No. 5,710,217 (Blong).